As a next-generation high breakdown voltage low loss switching element, there is hope for a vertical high breakdown voltage silicon carbide field effect transistor. This element, as shown in e.g., Patent Document 1, comprises well regions formed by a photolithography technique and an ion implantation technique, in a drift layer (second epi layer) existing on a silicon carbide substrate, in the vicinity of a surface of the substrate, a source region and a JFET region (current control region) existing below a gate electrode, being sandwiched between the pair of well regions.
Though one of means for improving the performance of the silicon carbide semiconductor device, especially the vertical high breakdown voltage silicon carbide field effect transistor, is miniaturization (e.g., reduction of cell pitch), it is preferable herein to also reduce the JFET length (the spacing between the pair of well regions below the gate electrode). With miniaturization, however, since the resistance of the JFET region which is fundamentally high sharply increases to raise the ON-resistance of the element, the drain current at ON operation decreases and this makes it impossible to improve the performance. Therefore, in order to achieve higher performance of this element, it is necessary to reduce the ON-resistance.
Among methods of reducing the ON-resistance are a method disclosed in Patent Documents 1 and 2 in which a double-layer structure consisting of drift layers having different impurity concentrations and a method disclosed in Patent Document 2 in which a current induction layer is provided in the JFET region.    Patent Document 1: Japanese Patent Application Laid Open Gazette No. 2000-286415    Patent Document 2: Japanese Patent Application Laid Open Gazette No. 2005-5578
In the high breakdown voltage low loss switching element, it is necessary to provide a field relieving region (JTE region) for relieving an electric field in a region in the vicinity of outer peripheral portion of a drift layer. When the impurity concentration of the drift layer is increased, however, in order to reduce the ON-resistance, the impurity of the drift layer affects the impurity concentration of the field relieving region. Therefore, it disadvantageously becomes difficult to appropriately determine an impurity concentration distribution of the field relieving region.